There are great differences in the way that different fluids or coatings wet out on and spread across solids. One fluid may spread out with ease on one solid but not at all on a different solid. This is due to the balance of surface energies for the system and its components. The contact angle is a measure of the degree to which a fluid wets a solid. The contact angle is measured through the fluid and is the angle between the surface of the fluid and the surface of the substrate as measured through the fluid at the contact line. Under dynamic conditions, if the angle is small, then the fluid wets and spreads across the solid readily. If it is large, then the fluid may not spread at all.
The contact angle that a fluid makes under static conditions is a measure of the relative surface energies of the solid and fluid. The contact angle that a fluid makes under dynamic coating conditions is a measure of how well the fluid "wets" or coats the substrate in competition or in concert with hydrodynamic and other forces. This is particularly true for fluids which are loaded with surface active agents, are coated on surface active solids or textured solids, or are coated with electrostatic assist, as in the photographic industry. In these environments, the observed dynamic contact angle differs from that which would be expected from a pure consideration of surface tension, hydrodynamic forces, and the contact angle measured under static conditions. Surfactants added to the fluid require a finite time to migrate to the surface and may or may not be present on the solution surface at the dynamic contact line. Also, surfactants may be present on the surface of the uncoated backing, the backing may have a distinct texture, and any electrostatic, magnetic, or other fields could affect the observed dynamic contact angle.
One measure of the competition between the surface tension forces and the hydrodynamic forces is the capillary number, which is the ratio of the viscous forces in a flow to the surface tension forces. At very low speeds and low capillary numbers the dynamic contact angle approaches the angle measured under static conditions. As the coating speed and the associated wetting speed increases, the contact angle increases. Eventually the contact angle reaches 180.degree. and air is entrained. This is reported to occur at a capillary number of approximately one in low viscosity fluids and is consistent with the expectation that surface forces dominate the flow at low capillary number, and that higher capillary numbers indicate increasing competition from hydrodynamic forces. With higher viscosity fluids, air entrainment can occur at higher capillary numbers.
The contact angle is an important factor in the investigation of low capillary number coating flows. In these flows, the coating bead shape is dominated by surface tension. The effect of the contact angle can dominate all other aspects of the flow. This is apparent when numerical modeling of the coating flows is attempted and a dynamic contact angle is specified as an input variable.
A number of techniques for measuring the dynamic contact angle are known. In the plunging tape method, the substrate being tested is plunged into a coating fluid bath. Substrate speed is increased until the critical velocity for air entrainment is reached and a tongue of air is pulled into the fluid to entrain air bubbles. The contact angle is measured by examining the vicinity of the contact line through the side of the tank as the speed changes. However, this method requires a large pool volume and a large testing surface area as compared with the region of interest. As the flow of the fluid outside of the vicinity of the contact line is not well defined, it may not represent the coating process. Also, the age of the fluid surface is uncertain, and may mask the effects of surface active agents. If the coating fluid contains volatile components, evaporation presents additional unknown effects. Additionally, the contact angle is difficult to view as measurements are made through the transparent side of the tank. If the fluid is opaque, no angle will be visible at all. Also withdrawal of the tape from the bath tends to pull a film of the coating bath with it. Data on the contact angle taken by this method shows scatter on the order of .+-.4.degree..
A second method of observing the contact angle involves pumping fluid through a capillary tube or between parallel plates. If this device is immersed in a fluid with an index of refraction similar to the substrate, the contact angle can be determined by a combination of microscopic examination and numerical approximation. This restricts investigation to surfaces which are substantially transparent. The size of the apparatus also limits the time available to take measurements and the speed of the substrate at which a measurement is practical.
A third method involves partially submerging a turning roll in a tank of fluid. The roll may or may not be doctored clean before its surface is rotated into the fluid to observe the contact angle. This method shows smaller contact angles and higher critical speeds than the plunging tape method. The pre-wet substrate and large tank volumes make the results obtained by this method of questionable value for the study of the coating process.
Other measurements of the dynamic contact angle for coating situations have been made using specially designed slot die coaters. In die-based coating systems, the fluid is metered out of the die nozzle and onto a substrate. The die is usually close to the substrate surface, the fluid has a well defined flow field and surface history, and the contact angle can be directly measured. However, when using a full size die coater, it is difficult to access the coating bead or to align the optics to get the bead of coating fluid in profile. Also, there is always an edge bead, which obscures the undisturbed coating bead. To avoid this edge bead, a sheet of glass is commonly mounted at the end of the slot coater and the coating bead is viewed from within the coating fluid. This is not practical with opaque fluids. Finally, the aperture of the optical system observes a plurality of light rays which cause multiple internal reflections. As a result, the dimensions of the bead to be examined must be relatively large to reduce obstruction.
These known systems are inconvenient to use, are limited in their choice of coating and substrate materials, and do not provide a fluid flow field which accurately simulates that experienced in a coater.